Plant Environmental Regulators - 1 Plant Responses to Environmental Signals Plants have many mechanisms to respond to conditions of their external environment, just as animals do. Plants routinely regulate growth and development activities by using environmental stimuli to trigger signal transduction pathways. For example, plants maintain internal “clocks”, demonstrate circadian rhythms, detect photoperiod changes and respond to many other environmental stimuli. Plants mobilize defense mechanisms in response to predator attacks, as we shall see at the end of this section. Often plant responses to environmental stimuli involve movement, something we rarely attribute to plants. There are a number of different types of movements of plants. For example: • Many plants exhibit spiral growth. Tendril twining is one example, discussed previously, and the pattern of bark growth on trees is a second. We can often see the latter on trees that have been hit by lightening. Responses that seem to be independent of the direction of the stimulus are said to be nastic movements. • Many plants have rhythmic movements controlled by photoperiodicity. These "circadian rhythms" often involve turgor changes. Closing leaves at dusk is one example of plant circadian rhythms. • Shoots and roots typically grow toward or grow away from environmental stimuli, such as light and gravity. Such directional movements are called tropisms. Positive tropisms are growth or movement towards the stimulus. Negative tropisms involve movement away from the stimulus. Nastic Movement Circadian Rhythm Directional Movement Plant movements may be responses to external conditions or to internal hormonal signals, which in turn may have been triggered by the external environment. Most tropisms involve differential elongation of cells; walls away from the stimulus grow differently than those closer to the stimulus. In some cases humans don’t yet know how plants detect and respond to changing environmental conditions.

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